In a number of devices, such as radiation imagers, it is desirable to apply a thin membrane to an irregularly shaped surface. For example, in a radiation imager in which a scintillator is optically coupled to a photosensor, it is desirable to apply a layer of reflective material to the surface of the scintillator opposite the surface adjoining the photodetector so that optical photons generated in the scintillator are reflected towards the photosensor. Commonly, one surface of the scintillator is irregularly shaped, that is, protrusions extend from the surface so that the surface is not flat. Such protrusions are needle or pyramid-like structures that result from the deposition process and serve to localize detection of photons generated in the array to the area in which the incident radiation was absorbed in the scintillator.
Application of reflective coatings to the irregular surface of the scintillator poses a number of difficulties. Many scintillator materials, such as cesium iodide, have large thermal expansion coefficients and thus are extremely sensitive to processes in which a reflective coating is deposited onto the surface, such as by sputtering. The relatively high temperatures associated with such deposition processes (e.g., above about 300.degree.-400.degree. C.) cause cracks in the scintillator material that create optical discontinuities and thus degrade scintillator performance.
For example, a reflective coating on a scintillator surface desirably significantly increases the number of optical photons detected by the photosensor as compared with an arrangement in which the scintillator has no reflective coating (e.g., an increase of at least one-third or more in optical photons captured is desirable). Further, the reflective coating should conform to the irregular shape of the scintillator so that optical photons are directly coupled between the reflective layer and the scintillator material, with few if any interstitial voids between the scintillator material and the reflective material. Additionally, the application of the reflective material should not degrade the scintillator structure, either by thermally degrading the material or mechanically deforming the needle or pyramid-like structure of the scintillator. Reflective materials having the desired optical and physical characteristics are available in monolithic thin membranes (as used herein, "monolithic" refers to a substantially uniform material in a sheet-like form), however application of such thin membranes to the irregular shaped surfaces of a scintillator without damaging the scintillator or the thin membrane has been problematic.
The monolithic thin membranes used to cover a scintillator and other optical components are typically quite thin (e.g., about 0.060" or less) and difficult to apply without damaging (e.g., tearing or bunching together) the thin membrane, or damaging the irregular surface to which it is applied (e.g., deforming some of the protrusions on the surface).
Conformal deposition of relatively thin membranes (or films) to irregularly shaped surfaces also has applicability in liquid crystal device fabrication and application of optical coatings to components such as lenses, reflectors, and the like.
It is thus an object of this invention to provide a device for depositing a thin membrane over an irregular shaped surface so that the membrane conforms to the irregular surface.
A further object of this invention is to provide a device for fabricating an imager having a thin membrane reflective coating disposed over a scintillator having an irregularly-shaped surface.
A still further object of this invention is to provide a device for disposing a thin membrane in a desired position with respect to a workpiece such that the membrane is precisely laterally aligned with the workpiece.